Sintering and Catalysis

Catalysts, commercial or laboratory made, show a decline in activity when they are used in reactions. This behaviour, detrimental to the industrial efficiency and often not well understood by the laboratory chemist, may result, in principle, from many reasons. One of the most suspected is the presence in the reactants of poisons which cover permanently the catalyst and compete therefore for the chemisorption of reagents. But also, the reaction products undergo a further evolution, like polymerization or cracking, and their deposit on the surface of the catalyst is comparable to poisoning. These phenomena, however, are to be distinguished from the sintering which is an additional, if not the main, cause of the loss in activity. The sintering may result in the decrease of the active area and in the change of the structure of the surface. For instance, the porous oxide catalyst or the porous support of the active metal may exhibit a narrowing or closing of its pores. The permeability of the catalytic bed decreases together with the loss of accessible area. The porous, high surface area oxide catalyst may also change its surface structure by surface recrystallization, and by formation or vanishing of surface defects, whose role may be important in catalysis.